The Dutch Safety Board issued a Safety Alert, warning aviation authorities about unexpected autopilot behavior on an ILS approach could lead to (approach to) stall conditions.

The Dutch Safety Board is investigating a severe and sudden pitch-up upset during an ILS approach to Eindhoven Airport in 2013. The airspeed dropped rapidly to a near stall situation in which the stick shaker activated. The crew carried out a go-around. During the investigation the Board became aware of similar events. Analysis revealed that the common factor linking these events is the ILS antenna type; M-array (Capture effect) ILS antenna. The
M-array ILS antenna type is used around the world, including at major airports and military air bases in the Netherlands.

Regulations mandate that ILS systems be periodically checked with a Flight Inspection in order to be certified for operational use. The Flight Inspection focuses exclusively on the 3 degree glide slope area. The signal characteristics in the area above the 3 degree glide slope were examined as part of the Dutch Safety Board’s investigation. Flight tests were conducted to measure the M-array antenna signal and determine the ‘glide slope field’ characteristics above the 3 degree glide path while established on the localizer.
Analysis of the measurements show that between the 3 and 9 degree glide path, signal strength changes. For the pilot this can result in observable movement of the ILS glide slope marker on the primary flight display. At this time two important characteristics of the M-array ILS antenna ‘glide slope field’ have been identified:
– A signal reversal was always present at approximately 9 degree glide path.
– A signal reversal was sometimes present at approximately 6 degree glide path.

Depending on the glide slope field, signal reversal occurs occasionally at 6 degree, and always at the 9 degree glide path. This reversal activates the glide slope capture mode after which the autopilot follows the glide slope signal without restrictions. During flight tests the reversal resulted in the automatic flight control system commanding a severe pitch-up. Immediate flight crew intervention was required to regain aircraft control.

Thus far the investigation has revealed that aircraft from four different manufacturers operated by different airlines have experienced a pitch-up upset caused by a false glide slope either under test conditions or during operation.
This investigative information has led the Dutch Safety Board to issue this Safety Alert to address the following safety concern: to generate awareness of different ILS signal characteristics and the potential of aircraft pitch-up upset due to capturing a false glide slope, which can lead to (approach to) stall conditions.